The invention relates to solar cells in general and more particularly to an improved solar cell and method of manufacture therefor.
Solar cells having zones of opposite doping forming a p-n junction in their semiconductor body, which zones are each provided with an electrode, are known.
Solar cells are electronic semiconductor components, by means of which sunlight can be converted into electric energy. The semiconductor body can consist, for instance, of silicon or a III-V compound such as gallium arsenide and is provided on its front side facing the radiation source with a p-n junction of large area by means of diffusion. Planar metal contacts on the back side and thin metallic contact strips on the front side are used as electrodes for collecting the current generated in such a semiconductor crystal. At the p-n junction a diffusion voltage, the magnitude of which is determined by the impurity concentration in the adjacent zones, is generated at thermal equilibrium. It forms an internal field over the space charge zone of the boundary layer. If light quanta with sufficiently large energy now enter such a semiconductor, additional pairs of charge carriers are produced on both sides of the p-n junction in excess over the thermal equilibrium. The charge carriers produced then move toward the p-n junction and are separated in the electric field of the latter. This separation results in a reduction of the internal potential. The difference from the potential of the thermal equilibrium appears as a photo voltage, the equalization of the charges then taking place in an external load circuit connected to the semiconductor crystal, giving off electric energy.
As is well known, solar cells are designed with a view toward permitting as many photons as possible to penetrate into the semiconductor and thus the number of charge carriers reaching the p-n junction as well as the available power become as large as possible. The zone of the semiconductor body facing the light source, which is in general of the n-conduction type which deteriorates less than a p-conduction zone, is therefore chosen as thin as possible, so that a high percentage of the light absorbed in the very thin semiconductor layer contributes to the energy conversion. The conversion length is then approximately equal to the diffusion length. In addition, the layer resistance of this n-conduction zone is chosen small lest the efficiency of the solar cell be reduced by a excessively large series or internal resistance. In addition, it is advantageous to choose a starting material with a resistivity of between 1 and 10 ohm-cm. The cells made of such materials are degraded but little if exposed to corpuscular radiation. Furthermore, the life of the minority carriers and, therefore, the diffusion length is sufficient large so that a considerable portion of the light quanta which are absorbed further inside on the side of the p-n junction facing away from the direction of the incident light, generate charge carriers which still can reach the p-n junction.
A large portion of the light incident on the semiconductor surface of a solar cell is reflected; in the case of a plane silicon surface, this portion can be as much as 32%. The known solar cells are therefore generally provided with a layer of suitable thickness of a material with a matched index of refraction, in order to limit the reflection losses to a negligible amount (German Offenlengungsschrift No. 1 934 751).
Solar cells generally contain a plane semiconductor body several hundred .mu.m thick, for instance, 350 .mu.m thick, of single crystal, p-conduction silicon, into the top side of which a thin n-conduction zone with a small thickness of, for instance, 0.3 .mu.m, is diffused. The manufacture of such silicon sheets, however, is very elaborate and expensive, so that the production of energy with such cells is substantially more expensive than other energy production methods.